Audio signal keyer

ABSTRACT

An audio signal keyer suitable for use in an electronic musical instrument comprises two differential amplifiers in cascaded connection. The first differential amplifier has a first input supplied with an audio signal to be keyed, a second input supplied with no AC-signal and a common end supplied with a keying voltage, whereby the first output gives out a keyed signal consisting of an AC-component building up with the determined envelope and a DC-component building up in accordance with the keying voltage variation and the second output gives out only a DC-component of the above-mentioned variation. The second differential amplifier has one input being supplied with the above-mentioned keyed signal of the AC- and DC-components appearing at the first output of the first differential amplifier and the other input being supplied with the DC-component appearing at the second output of the first differential amplifier. Thereby, at one of the outputs of the second differential amplifier is obtained a keyed audio signal with only the AC-component building up the desired amplitude with the determined envelope, which is free from the unwanted DC variation and click noise occurring at the beginning of the tone sounding.

.nited States Patent 1191 Nagahama 1 AUDllU SIGNAL KEYER [75] Inventor: Yasuo Nagahama, Hamamatsu,

Japan [73] Assignee: Nippon Gakki Seizo Kabushiki Kaisha, Hamamatsu-shi, Shizuoka-ken, Japan [22] Filed: Aug. 115, 1973 [21] Appl. No.: 388,621

[52] US. Cll 84/1111, 84/DIG. 23, 84/124,

84/ 1.26 [51] lint. C1. Gllilh 1/00 [58] Field of Search 84/1.01, 1.24, 1.26, 1.14,

84/D1G. 7, DIG. 8-, DIG. 23; 307/239, 246, 248

[56] References Cited ,1

UNITED STATES PATENTS 3,602,626 8/1971 Aramaki 84/1.l3 3,637,915 1/1972 Hirano V 3,656,008 4/1972 Nakagomi.............. 3,665,091 5/1972 Hir ano 84/1 13 3,746,775 7/1973 Uchiyama 84/l.l3

Primary Examiner-Richard B. Wilkinson Assistant ExaminerW. Weldon Attorney, Agent, or F irm-Cushman, Cushman Darby &

[57] ABSTRACT An audio signal keyer suitable for use in an electronic musical instrument comprises two differential amplifiers in cascaded connection. The first differential amplifier has a first input supplied with an audio signal to be keyed, a second input supplied with no AC-signal and a common end supplied with a keying voltage, whereby the first output gives out a keyed signal consisting of an AC-component building up with the determined envelope and a DC-component building up in accordance with the keying voltage variation and the second output gives out only a DC-component of the above-mentioned variation. The second differential amplifier has one input being supplied with the above-mentioned keyed signal of the AC- and DC- components appearing at the first output of the first differential amplifier and the other input being supplied with the DC-component appearing at the second output of the first differential amplifier. Thereby, at one of the outputs of the second differential amplifier is obtained a keyed audio signal with only the AC- component building up the desired amplitude with the determined envelope, which is free from the unwanted DC variation and click noise occurring at the beginning of the tone sounding.

8 Claims, 9 Drawing Figures PATENTEUJUL 91914 3, 823 0244 saw 1 or 2 FIG. I PRIOR ART PATENTED JUL 74 SHEU 2 BF 2 mEmCUE mZOP O.

mmObEwZmO mZOP 20mm BACKGROUND OF THE INVENTION 1. Field of the Invention:

The present invention relates generally to an audio signal keyer and more particularly to improvements in an audio signal keyer for use in an electronic musical instrument, which is free of click noise occurring upon keying operation of the instrument.

2. Description of the Prior Art:

Generally, electronic musical instruments or other key-actuated electrical musical instruments have employed tone keyers for keying tone signals from tone generators in order to not only give the signals required keying envelopes (amplitude characteristics) but also eliminate click noise accompanying the keyed-tone signals due to the abrupt changes of the envelope at the key-on and key-off operation. As a keyer of the type such that a tone signal is keyed on and off by controlling the biasing of a transistor amplifier one uses an emitter follower circuit in which keyer is effected on the emitter side and the other uses a grounded emitter amplifier circuit. The latter type is also provided with a key-switch on its emitter side and is advantageous in that it has voltage amplifying function when a low level input signal is keyed. As described hereinafter, however, the keyed tone signal appearing at the output has a DC variation superimposed on an audio signal keyed with a desired envelope. The DC variation occurring at the transient of keying-on results in a click noise and an unpleasant sound signal. These inconveniences and shortcomings result from not only mere superimposition of the DC variation on the keyed tone signal, but also the fact that due to the addition of the DC component the loudspeaker diaphragm vibrates in the state of being biased in one direction thereof and thereby radiating sounds in a high distortion area of the diaphram. An attempt to reduce such a DC biasing has been made by passing the DC-superimposed keyed tone signal through a circuit (low-cut filter) composed of a capacitor and a resistor with appropriate circuit constants. However; this attempt also is not preferable because it causes unwanted variations in spectrum of the tone signal, namely, variations in the tone color of the produced sound, and also the click noise can not be suppressed sufficiently.

Especially, when a bass signal such as a pedal sound in the electronic musical instrument is keyed, no such click noise can be eliminated even by using such as a filter since tone signals per se are generally of a low frequency range.

In addition, the use of the capacitor in the signal path for blocking the DC component is inconvenient for integration of circuit in view of the fact that a large number of such audio keyers are employed in an electronic musical instrument. In other words, because of the difficulty of manufacturing capacitors within an integrated circuit form, such conventional audio keyers are not satisfactorily cheaper to manufacture and reliable in operation.

SUMMARY OF THE INVENTION It is therefore the primary object of the present invention to provide an audio signal keyer which permits stable operation and is free of click noise occurring at the transition of a key-switch by manipulation.

Another object of the present invention is to provide an audio signal keyer which has a novel construction capable of preventing the keyed signal from being modulated with variations in a DC component and from being biased with the DC potential.

A further object of the present invention is to provide keyer means in combination with a key-actuated electronic musical instrument for creating tonal effects without adversely affecting the mood of the music being played and with reliability.

A still further object of the present invention is to provide an arrangement of a keyer which is relatively easy for circuit integration when a large number of keyers are necessary to use as in a key-actuated electronic musical instrument.

In accordance with the present invention, there is provided a novel audio signal keyer means which can completely eliminate the prior art defects mentioned above and comprises a first differential amplifier responsive to an audio signal as the input of the keyer for developing at paired different output terminanls thereof the DC-superimposed keyed audio signal through an amplitude-characteristics producing means and a DC signal corresponding to an average potential of the keyed audio signal respectively, and a second dif ferential amplifier having a pair of input terminals connected with the paired output termianls of the first differential amplifier respectively for rejecting the DC sig nal from the DC-superimposed audio signal whereby at one of the output ends-of the second differential amplifier only the keyed audio signal without the DC component may be derived as the output signal of the keyer, namely, no other component than the keyed audio signal contains in the buildup amplitude characteristic of the keyed output signal.

In another aspect of the present invention, the present keyer system for use with an electronic musical instrument comprises first plural differential amplifiers responsive to different audio input signals for developing on the output sides the keyed audio signals and a common DC change signal depending upon an average potential of the keyed signalsthrough respective amplitude characteristic modifying means, and a second single differential amplifier supplied with the respective keyed signals derived from the first differential amplifiers at multi-input termianls and the DC change signal at a common input terminal, thereby deriving at multioutput terminals of the second differential amplifiers the keyed signals not containing the DC component as output signals of the keyers.

Other objects, features and advantages of the inven tion will become more apparent from the following detailed description of the preferred embodiments when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic'circuit diagram of a conventional audio signal keyer.

FIGS. 2A, 2B and 2C are charts of waveforms for explaining the operation of the conventional keyer circuit of FIG. ll.

FIG. 3 is a circuit diagram of an audio signal keyer incorporating therein a pair of differential amplifiers in accordance with one embodiment of the present invention.

; ing through the resistor R The resistance value of the resistor R is selected to be small so as to make the buildup time of the discharge shorter and accordingly, Q the time constant for discharge is also small. With the i beginning of discharge of the capacitor C the transis- 35 superimposed on the varying DC potential signal, so

1 nal applied to the input of the transistor keyer is ampli- 40 fied by the transistor Q and is developed at the transis- I tor output end a in a superimposed form on the DC po- FIGS. 4A, 4B and 4C are waveform charts for using the explanation of operation of the present keyer circuit illustrated in FIG. 3.

FIG. 5 is a schematic circuit diagram of a multi-keyer system illustrating an embodiment of application of the 5 present invention in a key-actuated electronic musical instrument.

Throughout the specification, like reference numerals or letters indicate like parts or components hereun; der.

Referring now to FIG. 3 an embodiment of the present invention will be described which is intended to overcome the prior art defects and comprises a pair of differential amplifiers in construction.

The audio signal keyer of this embodiment includes an input terminal IN adapted to be supplied with an audio signal to be keyed from for example, a tone generator, transistor base biasing resistors Rb, and R12 collector load resistors Rc Rc RC3 and RC4, emitter 'resistors Re R6 an input resistor R resistors R R.,, 3 11295 Ri a b rae i s ar s aasi q C yoperated on-off switch S. transistors Q to Q basically fconstituting two differential amplifiers, and an output ltern inal OUT of the keyer.

DESCRIPTION or THE PREFERRED EMBODIMENTS For easy and good understanding of the p resent iri-l l5 vention prior to the description of the preferred em-: bodiments thereof an example of the prior art using a grounded emitter circuit which is considered to be close to the present invention will be explained with reference to FIGS. 1, 2A, 2B and 2C. I

FIG. I illustrates a circuit arrangement of a conventional grounded-emitter type keyer, which comprises an input terminal IN, resistors R to R capacitors C C,, a key-operated switch S, a transistor 0 and an out- 2 5 put terminal OUT. In the keyer circuit, when a rectangular waveform tone signal as shown in FIG. 2A is applied to the input terminal IN and the key;, operated switch S normally open is now closed, the normally charged capacitor C is now shunted to begin discharg-v ferential amplifier including the input resistor R col- ?lector load resistors Rc, andRc a pair of transistors Q and Q base biasing resistors Rb, and Rbz for the respective transistors and the emitter resistor Rm; a keying voltage developing circuit including the key operated on-off switch S,.the capacitor C" and resistors R4 and R and determining the signal buildup time and decay time thereby controlling the emitter voltages of the transistors Q and Q by the voltage across the capacitor Co which is rendered to charge or discharge depending upon the on-off operation of the key-operated on-off switch S; a second differential amplifier including the collector resistors RC3 and R0,, another pair of transistors Q and Q and the common emitter resistor Rg and connected with the first differential amplifier to be supplied with two output signals of the first differential amplifier as its two inputs; and a power source circuit including a DC voltage divider having the three resistors R R and R for dividing a DC line voltage of l 2 volts into different po- EIGILIIQSMIIQLLQISSDEPlICd respectively to the bases of the transistors Q and Q jdeveloping circuit.

tor 0 begins to be conducting. The DC potential at the' collector electrode a of the transistor Q changes as shown by Vca in FIG. 28. With the change in the DC potential of point a, the rectangular waveform tone sigand to it? keyihfioltage be described with particular reference ot FIGS. 2A and 4A to 4C. When the key-operated. on-off switch S is rendered open, the capacitor C is charged with the potential supplied from the power source (4 volts between 8V and 12) at a time constant determined by the resistance of resistor R and the capacitance of capacitor C until the potential across the capacitor C reaches the potential of, for example, 8 volts at the tential Vca. That is, the rectangular waveform signal is that due to a change in the DC potential signal click noise and other unwanted sound signals result, as described previously. In order to reduce the DC drift of the rectangular waveform signal, the capacitor C l and the resistor R, (a kind of low-cut filter) are connected between'the transistor output end a and the output terminal OUT of the keyer by settling the circuit constants of these capacitor and resistor to lower the variation of the DC signal. The insertion of the capacitor C and the resistor R however, also causes a variation in the spectrum or the tone color-of the amplified tone signal disadvantageously. More in detail, in the transient of charging on the capacitor C the DC potential Vcout at the point OUT varies as illustrated by a DC waveform in FIG. 2C, and accordingly, the superimposed tone signal also varies as viewed from the waveform in FIG. 2C, so that inconveniences due to the change of the DC potential Vcout are not avoidable and click noise cannot be also prevented. Particularly, even the abovementioned filter composed of capacitor C and resistor R becomes impossible to reject only such click noise when an extremely low-frequency tone signal such as generated by pedal depression in the instrument is to be keyed.

0 .ing the value of an average potential of the input audio signal supplied to the keyer appropriately in connection with the charged potential of the capacitor C the paired transistors Q and Q constituting the first differential amplifier can be rendered to be non-conductive,

5 so that no audio signal appears on the output side,

namely, at the collector ends a and b.

On the other hand, when the key-operated on-off switch S is rendered closed the charge stored on the capacitor C begins to discharge through the shunt resistor R and hence the emitter potential of the paired transistors Q, and Q varies from 8 volts to l 2 volts, so that these transistors Q, and Q are rendered conductive. At this time, if an audio signal having a rectangular waveform as illustrated in FIG. 2A is applied to the input terminal IN, an output signal as shown in FIG. 4A develops at the point a on the collector electrode of the transistor Q, whereas a DC voltage signal with a variation as shown in FIG. 48 appears at the point b on the collector electrode of the transistor Q.

I "More in detail, the keyer circuit comprises aTIYst dif Operation of the present audio signal keyer will now- 5 junction h between the resistors R and R By select- The signals at the output points a and b of the first differential amplifier are supplied to the base electrodes of the other paired transistors Q and Q constituting the second differential amplifier, by which the DC components of the same phase which are contained in both the supplied signals are cancelled or compensated by each other. Consequently, at the output end OUT of the transistor O is obtained an audio signal having a rectangular waveform as illustrated in HQ. 4C, which does not contain any variation in the DC component. The thus produced audio signal can be utilized as the keyed output signal for an electronic musical instrument for example.

Referring now to FIG. 5, there is illustrated diagrammatically a keying system of a key-actuated electronic musical instrument employing a plurality of threeinputs keyers of the construction described above by way of example.

in the drawing, blocks designated by (3, c D, D ti B represent the input part of keyers for the respective notes in an octave. Each block comprises a first differential amplifier constituted by three left transistors and one right transistor and three input terminals 4i, 8 and 116 which are adapted to be supplied with audio signals from tone generators. The blocks are equal in number to that of the keys of the instrument and comprises four-grouped transistors QC QC QC QC #4,. QB, QB4 of which each group constitutes a three-input differential amplifier having three-inputs and one-reference, charge-discharge capaciwrs C0 siates FQGHQLEYQEEEQQEQ? switches S Sn, whereby the emitter bias of the transistor differential amplifier is controlled to make it conducting and non-conducting depending upon the variation in the potential across each charge-discharge capacitor C Block designated by'O includes four transis- I tors constituting a second differential amplifier also constituted by three left transistors and one right transistor. The second differential amplifier comprises three input transistors O0 Q0 and Q0 respectively connected, in common for all notes, to the respective output terminals of each first differential amplifier and a common input transistor Q0, whose input is supplied with the DC component in common from the first differential amplifiers, and which can accomplish a differential amplifier function with the respective input transistors O0 Q0 and Q0 At the collector electrodes of these transistors O0 Q0 and Q0 are provided three output terminals OUT of the keyers corresponding to 16-foot, 8-foot and 4-foot register notes respectively, which may be connected to the next stage such as tone coloring filters.

In operation, when the key-operated on-off switch S, associated with the keyer block corresponding to notes C of the musical scale is closed, the charge stored in the capacitor C is shunted through the resistor R to cause discharge with a time constant which is determined by the resistance of resistor R and the capacitance of capacitor C with the result that nonconducting transistors 0 Q, are rendered conducting. Hence, audio input signals representative of musical scale tones for 4-foot, 8-foot and 16-foot registers from the tone generators are delivered to the bases of the transistors Q0 Q0 and a DC component corresponding to an average potential of these audio signals is supplied to the common transistor Q0 so that at the corresponding output terminals OUT of the second differential amplifier O are derived audio signals which are free of DC drifting for the reason stated above.

When the on-off switch S is turned open, the capaci tor C begins to charge with a time constant which is determined by the resistance of resistor R and the capacitor C to thereby lead the transistors QC, QC, in the non-conducting state. As a result, the delivery of the audio signals toward the output differential amplifier is interrupted.

Also, when a plurality of keys are depressed simultaneously to make corresponding plural key-operated switches closed so as to produce chord, the keyers operate in a similar manner as described above. At this time, audio input signals are mixed and fed to the three output terminals l6, 8 and 4- of the keying system.

It will be appreciated that since the present keyer arrangement is formed of almost transistors and resistors, a keying system requiring a large number of keyers is easy to be manufactured in an integrated circuit form.

As described in the foregoing, the present invention employs an input differential amplifier so arranged as to enable keying action with the addition of any amplitude characteristics and an output differential amplifier coupled-with the outputs of the input differential amplifiers to reject the DC component which is superimposed on an audio output signal of the input differential amplifier, so that click noise caused at the signal buildup by keying can be eliminated.

Such an arrangement provides great industrial merits particularly for use in an electronic musical instrument in that audio signals to be keyed can be passed therethrough without being modified with undesired mechanical and electrical factors over the whole frequency range of the signals.

l'claim:

l. An audio keyer comprising a first differential amplifier having one input terminal to be supplied with an audio signal to be keyed and another input terminal to be supplied with a DC reference potential and a control terminal; a keying voltage developing circuit including a key-operated charge-discharge element and connected to said control-terminal of said first differential amplifier so as to control the bias of the amplifier to thereby develop a keyed audio output signal at one of the output terminals of the amplifier and develop a DC potential corresponding to said average potential at the other terminal; and asecond differential amplifier having one input terminal connected directly to said one output terminal of the first differential amplifier and another input terminal connected directly to the other output terminal of the first differential amplifier and having one output terminal for deriving a difference signal between the two inputs such that a DC component superimposed on the keyed audio signal is eliminated.

2. The keyer according to claim ll, which comprises DC voltage divider means having dividing resistors for supplying predetermined potentials to said first differential amplifier and said charge-discharge element, respectively.

3. The keyer according to claim 1, in which said charge-discharge element is a capacitor.

4. The keyer according to claim 1, in which each of said first and second differential amplifiers includes a pair of transistors having emitters connected in common, bases connected with said two input terminals and collectors capable of deriving outputs.

5. The keyer according to claim 4, in which said common emitter of the first differential amplifier is connected through said control terminal to the chargedischarge element across which the potential charges depend on the action of a key-operated on-off switch.

6. The combination in an audio signal keyer comprising a first differential amplifier including an input resistor, a pair of transistors first one of which has a base supplied with an audio signal through the input resistor and a collector adapted to develop the supplied audio signal across a collector load resistor and second one of which has a base connected to a bias resistor and a collector capable of developing a DC signal across another collector load resistor, emitters of both said transistors being connected through an emitter resistor; signal buildup time and decay time determining means including a key-operated on-off switch and a chargedischarge capacitor in parallel connection between a DC voltage line and the common emitter of the first differential amplifier and reisitors for determining chargedischarge timing with said capacitor; a second differential amplifier including paired third and fourth transistors having bases directly connected with the collectors of the first and second transistors of the first differential amplifier and emitters connected to each other, said third transistor having a collector connected to another collector load resistor for deriving a difference output signal; and power supply means including a DC voltage divider having three resistors for dividing said DC power voltage into predetermined potentials for the base bias voltage of the first differential amplifier transistors and the charge-discharge capacitor.

7. The combination according to claim 6, in which the DC signal developed at the collector of the second transistor of the first differential amplifier -is predetermined to be equal to an average voltage of audio signal developed at the collector of the first transistor by setting the base bias voltage of the second transistor in connection with the potential across the chargedischarge capacitor.

8. A keying system for an electronic musical instrument comprising a large number of keyers equal in number to that of playing keys, each keyer including a plurality of input terminals adapted to be supplied with tone signals; a plurality of first input transistors connected to said plurality of input terminals respectively at their bases for keying the tone signals at their collectors; a reference DC voltage generating transistor constituting first differential amplifiers with said plurality of transistors, the emitters of the amplifier transistors being connected in common; and a key-operated signal amplitude characteristic determining circuit having a charge-discharge capacitor and resistors and varying the potential at the capacitor depending upon on-off action of a key-operated switch, said capacitor being connected in series with the common emitter of the first differential amplifiers so that the emitter bias may be controlled by the potential variation of the capacitor, and second differential amplifiers provided in common for the whoe keyers and for deriving keyed output signals, said second amplifiers including a second plurality of input transistors equal in number to that of the input transistors of each keyer and the bases of said second input transistors being connected with respective collectors of said first input transistors of each keyer and a DC reference transistor constituting differential amplifier pairs with said second input transistors and the base of the said DC reference transistor is connected to the collector of the said reference DC voltage generating transistor in the first differential amplifiers, whereby keyed output tone signals derived from the second amplifiers are free from the DC component. 

1. An audio keyer comprising a first differential amplifier having one input terminal to be supplied with an audio signal to be keyed and another input terminal to be supplied with a DC reference potential and a control terminal; a keying voltage developing circuit including a key-operated charge-discharge element and connected to said control terminal of said first differential amplifier so as to control the bias of the amplifier to thereby develop a keyed audio output signal at one of the output terminals of the amplifier and develop a DC potential corresponding to said average potential at the other terminal; and a second differential amplifier having one input terminal connected directly to said one output terminal of the first differential amplifier and another input terminal connected directly to the other output terminal of the first differential amplifier and having one output terminal for deriving a difference signal between the two inputs such that a DC component superimposed on the keyed audio signal is eliminated.
 2. The keyer according to claim 1, which comprises DC voltage divider means having dividing resistors for supplying predetermined potentials to said first differential amplifier and said charge-discharge element, respectively.
 3. The keyer according to claim 1, in which said chargedischarge element is a capacitor.
 4. The keyer according to claim 1, in which each of said first and second differential amplifiers includes a pair of transistors having emitters connected in common, bases connected with said two input terminals and collectors capable of deriving outputs.
 5. The keyer according to claim 4, in which said common emitter of the first differential amplifier is connected through said control terminal to the charge-discharge element across which the potential charges depend on the action of a key-operated on-off switch.
 6. The combination in an audio signal keyer comprising a first differential amplifier including an input resistor, a pair of transistors first one of which has a base supplied with an audio signal through the input resistor and a collector adapted to develop the supplied audio signal across a collector load resistor and second one of which has a base connected to a biaS resistor and a collector capable of developing a DC signal across another collector load resistor, emitters of both said transistors being connected through an emitter resistor; signal buildup time and decay time determining means including a key-operated on-off switch and a charge-discharge capacitor in parallel connection between a DC voltage line and the common emitter of the first differential amplifier and reisitors for determining charge-discharge timing with said capacitor; a second differential amplifier including paired third and fourth transistors having bases directly connected with the collectors of the first and second transistors of the first differential amplifier and emitters connected to each other, said third transistor having a collector connected to another collector load resistor for deriving a difference output signal; and power supply means including a DC voltage divider having three resistors for dividing said DC power voltage into predetermined potentials for the base bias voltage of the first differential amplifier transistors and the charge-discharge capacitor.
 7. The combination according to claim 6, in which the DC signal developed at the collector of the second transistor of the first differential amplifier is predetermined to be equal to an average voltage of audio signal developed at the collector of the first transistor by setting the base bias voltage of the second transistor in connection with the potential across the charge-discharge capacitor.
 8. A keying system for an electronic musical instrument comprising a large number of keyers equal in number to that of playing keys, each keyer including a plurality of input terminals adapted to be supplied with tone signals; a plurality of first input transistors connected to said plurality of input terminals respectively at their bases for keying the tone signals at their collectors; a reference DC voltage generating transistor constituting first differential amplifiers with said plurality of transistors, the emitters of the amplifier transistors being connected in common; and a key-operated signal amplitude characteristic determining circuit having a charge-discharge capacitor and resistors and varying the potential at the capacitor depending upon on-off action of a key-operated switch, said capacitor being connected in series with the common emitter of the first differential amplifiers so that the emitter bias may be controlled by the potential variation of the capacitor, and second differential amplifiers provided in common for the whoe keyers and for deriving keyed output signals, said second amplifiers including a second plurality of input transistors equal in number to that of the input transistors of each keyer and the bases of said second input transistors being connected with respective collectors of said first input transistors of each keyer and a DC reference transistor constituting differential amplifier pairs with said second input transistors and the base of the said DC reference transistor is connected to the collector of the said reference DC voltage generating transistor in the first differential amplifiers, whereby keyed output tone signals derived from the second amplifiers are free from the DC component. 